The present invention relates to a thermal treatment apparatus and, in particular, to a thermal treatment apparatus used in the process of fabricating a semiconductor device.
During the conventional process of fabricating a semiconductor device, an apparatus such as a CVD apparatus, oxide film formation apparatus, or diffusion apparatus is used to perform an operation such as form a film or an oxide layer on a semiconductor wafer, or implant impurities. One such apparatus is a thermal treatment apparatus wherein semiconductor wafers are inserted into a reaction vessel by means such as a quartz boat capable of holding a plurality of semiconductor wafers horizontally, and a treatment is performed thereon by a reaction gas introduced into the reaction vessel that has been heated to a high temperature. This thermal treatment apparatus can be used in many different ways to simultaneously and rapidly treat a plurality of semiconductor wafers, by adjusting the temperature and heating.
However, as semiconductor devices have become smaller and also the diameters of semiconductor wafers have increased, there is demand for forming an ultra-thin film on a semiconductor wafer of a large area. The thermal treatment apparatus can perform batch processing in which a plurality of semiconductor wafers are processed at the same time, but unevenness in processing can easily occur between the central and peripheral portions of the same semiconductor wafer, and between semiconductor wafers at different positions within the thermal treatment apparatus. Various improvements have been proposed in the past to prevent this unevenness.
A thermal treatment apparatus of the above type is provided with a vertical cylindrical process tube made of a heat-resistant material such as quartz, and heaters for heating the interior of the process tube to a high temperature of 900.degree. C. to 1200.degree. C., or even higher, are arranged around the process tube. The lower part of the process tube is formed as an opening through which a heat-resistant quartz boat in which a plurality of semiconductor wafers are supported horizontally can be conveyed. The opening is provided with a lid body that has a contrivance such as an 0-ring for maintaining the interior airtight. The lid body is supported by a support body that is connected to an elevator drive mechanism with coil springs therebetween, so that the opening of the process tube can be opened and closed thereby. A heat-insulating tube is provided in the lid body, and the entire body of the quartz boat that supports the semiconductor wafers mounted on the heatinsulating tube is positioned in a flat heating zone of the process tube in such a manner that the semiconductor wafers are heated uniformly and are thus treated uniformly. In addition, a circulation path for cooling water is provided in the lid body so that the lid body is cooled and thus apparatuses below the process tube are prevented from being heated.
In order to introduce reaction gas into this process tube, a reaction gas introduction pipe connected to a reaction gas supply body is provided from below the process tube to an upper portion thereof along to the inner wall, and the reaction gas which is heated while it is passing through the introduction pipe is supplied from a supply opening provided .in an upper portion of the process tube. The arrangement .is such that reaction products created while the semiconductor wafers are being treated are discharged from an exhaust opening which is provided above the process tube and is connected to an exhaust apparatus, together with excess reaction gas.
The thermal treatment apparatus configured in this manner is also provided with a rotation mechanism that rotates the quartz boat of semiconductor wafers to ensure that the treatment effected within each semiconductor wafer is uniform. The rotation mechanism is configured so as to support the heat-resistant tube on a shaft connected to a motor shaft provided on the outside of the process tube.
Unfortunately, reaction products tend to adhere to the lid body and the inner wall at the bottom of the process tube of this conventional thermal treatment apparatus. In particular, when a doping gas such as POC1.sub.3 is used in a diffusion apparatus, phosphorous glass fused from phosphorous and SiO.sub.2 cannot be prevented from depositing on these areas. There is a danger that adhered substances layered in this way may later peel off and fly into the process tube, contaminating the semiconductor wafers and deteriorating the yield thereof. The adhered substances may also peel off while the semiconductor wafers are being conveyed into or out of the process tube, forming a source of contamination of the clean room.
If the thermal treatment apparatus is provided with a rotation mechanism, there is the danger that reaction products may adhere in the small gap which is formed between a hole in the lid body and the rotational shaft connected to the shaft of the motor, and which is necessary to allow rotation. In the worst case, the reaction products may peel off while the quartz boat is rotating, to form contamination sources.